1. Technical Field
The present invention relates to a switching power supply apparatus which functions as an insulation-type converter which converts an input voltage into a desired voltage for supply to electronic equipment.
2. Description of the Related Art
(Single-Ended Forward Converter)
Conventionally, as an insulation-type converter capable of converting an input voltage into a predetermined voltage to obtain an output voltage, a single-ended forward converter has been known (Patent Document 1).
As depicted in FIG. 17, a single-ended forward converter 100 is configured so as to have an input power supply 102 to which a primary winding 104a of a main transformer 104 and a switching element 106 are connected. To a secondary winding 104b of the main transformer 104, a rectifying circuit 107 including a forward-side synchronous rectifying element 108 and a flywheel-side synchronous rectifying element 110 and a smoothing circuit 111 including an output choke coil 112 and an output capacitor 114 are connected. To the output capacitor 114, a load is connected. Thus, output electric power can be obtained.
A control circuit 116 is a PWM control circuit which generates a fixed switching frequency, and ON/OFF of the main switching element 106 is controlled.
ON/OFF of the forward-side synchronous rectifying element 108 and the flywheel-side synchronous rectifying element 110 is controlled by a synchronous rectifying element drive circuit 122 including a switching element 126 which operates upon receiving a voltage generated at a tertiary winding 104c of the main transformer 104, a diode 128, and a Zener diode 130 and a gate discharge circuit 120 including a switching element 126 and a signal transmission transformer 124.
The forward-side synchronous rectifying element 108 is turned ON/OFF in synchronization with ON/OFF of the main switching element 106. The flywheel-side synchronous rectifying element 110 is turned ON when the main switching element 106 is in an OFF state, and is turned OFF when the main switching element 106 is in an ON state. OFF of the flywheel-side synchronous rectifying element 110 and ON of the main switching element 106 (corresponding to ON of the forward-side synchronous rectifying element 108) are driven, with deadtimes provided by a delay circuit 118.
Electric power supplied from the input power supply 102 is supplied to the main transformer 104 as interrupted electric power by ON/OFF of the main switching element 106. The primary winding 104a and the secondary winding 104b of the main transformer 104 are used in forward coupling, and electric power when the main switching element 106 is in an ON state is transmitted from the primary winding 104a to the secondary winding 104b. 
By the rectifying circuit 107 and the smoothing circuit 111 rectifying and smoothing electric power transmitted through forward coupling in the main transformer 104, an output electric power Vo is generated. The output voltage Vo is determined by an input voltage Vin, a turns ratio between wire turns N1 of the primary winding 104a and wire turns N2 of the secondary winding 104b in the main transformer 104 (N2/N1), and on-duty duty of the main switching element 106.Vo=(N2/N1)·Vin·duty
The single-ended forward converter 100 operates with a fixed switching frequency and, since the output voltage can be controlled by on-duty of the main switching element 106, can be operated with a simple control circuit (PWM control circuit). Also, since the main transformer 104 is used as a forward transformer, conversion efficiency of the transformer is high.
(LLC Resonant Converter)
Conventionally, as another type of the insulation-type converter capable of converting an input voltage into a predetermined voltage to obtain an output voltage, an LLC resonant converter is present (Patent Documents 2 and 3).
FIG. 18 depicts a conventional LLC resonant converter (Patent Document 2). To a primary winding N1 of a main transformer T1 provided to the LLC resonant converter, a serial resonant circuit including a resonant capacitor C1 and a resonant inductor L1 is connected. Switching elements Q1 and Q2 form a half-bridge circuit in which the elements are symmetrical with a duty of approximately 50% and are alternately turned ON/OFF, and switching frequencies of the switching elements Q1 and Q2 are controlled so that the resonant capacitor C1, the resonant inductor L1, and the main transformer T1 are in a resonant state.
The structure is such that a full-wave rectifying circuit of a center tap type including rectifying elements D3 and D4 and a smoothing circuit including an output capacitor Co are connected to a secondary winding N2 of the main transformer T1 and a load is connected to the output capacitor Co.
A control circuit 202 which drives the switching elements Q1 and Q2 is a frequency modulation control circuit (PFM control circuit), and the switching frequency is controlled so that the output voltage has a predetermined value by control of a VCO (voltage controlled oscillator) in accordance with an error voltage between an output voltage by a differential amplifier 203 and a reference voltage. Here, the switching frequency is controlled so that the resonant capacitor C1, the resonant inductor L1, and the main transformer T1 are always in a resonant state.
Unlike the single-ended forward converter depicted in FIG. 17, the LLC resonant converter is free from an increase in switching loss, generation of surge voltage, and so forth because the switching elements Q1 and Q2 are driven by soft switching operation. Therefore, a switching power supply with high efficiency and low noise can be obtained.